The official story is that General de Marmont by order of Napoleon build this pyramid of sods and earth in 27 days with the help of many soldiers. This is in itself is quite plausible, as the pyramid is only 36 meters high.
However there are pictures from around 1805, which depict a stone pyramid structure, and not of sods and earth.
Supposedly Napoleon and Marmont got the idea for building this pyramid after they visited the Giza pyramids.
The question is why would they build a pyramid somewhere in a forest in The Netherlands (and not in Paris for instance)? Official explanation: they needed a high vanatage point against attacks from the english (in the middle of the Netherlands?!?!)
Some suspect they didn't build this pyramid but rather uncovered it in the forest in the Netherlands. And afterwards the pyramid was covered by earth and sods.
Unfortunately there is not much research done on this and there are much more questions to be asked.
Like why are there still steps to be seen after 200 years of rain when it would be build out of sods and earth?